The technical field to which this invention relates is that of digital communications, in particular wireless communications. The systems to which the invention mostly relates are multicarrier systems and spread spectrum systems or any system formed from the combination of these two systems, including MC-CDMA.
Wireless communications are used for example in cellular networks, in a local radio loop and in RF broadcasting.
The exponential growth in wireless communications makes it crucial to optimize the use of the RF spectrum, a rare and expensive resource.
The invention proposes to contribute to this optimization in two ways:                by improving the transmitter/receiver link budgets, thereby making it possible to extend the radio coverage for given transmission power. However, the power transmitted is generally limited by the technology (linearity, etc) and the cost of the amplifiers, also by the energy consumption (battery, etc)        by increasing the capacity of cellular radio systems, itself dependent on the quality of the links.        
Transmit antenna diversity is a means of carrying out this optimization by exploiting the properties of the propagation channel linking the transmitter to the receiver.
In the field of mobile telephony, the propagation channel between the base and the mobile exhibits a random character, caused by the recombining of the multipaths created by reflections off buildings and various obstacles present in the environment.
Frequency selective fadeouts thus occur and may impair the quality of the communications. Two antennas are said to be in diversity when the channels linking them to the same mobile are statistically uncorrelated.
Diversity can be obtained either by placing the antennas a sufficient distance apart, or by using differently polarized antennas, or by diverse polarization accesses of the same physical antenna, or by any other means.
Receive antenna diversity has been known for a very long time as a means of combating these fadeouts. Specifically, if the waves picked up are statistically uncorrelated, this will make it possible through diverse processing means: selection, combination, etc . . . to obtain less frequent and/or less severe decays. One therefore obtains a first gain afforded by the presence of several receivers (the power received is multiplied accordingly), and a second due to the above processing. This second gain may be substantial [1], and is correspondingly bigger for very low error rates.
Transmit diversity at the base station is more expensive than receive diversity, since it requires two transmitters, for example one at each of the antennas. The benefits stem from the same origin: transmitting the signal via two statistically uncorrelated paths. However, the first of the two gains cited above for receive diversity disappears: there is now just one receiver. Only the second gain remains.
Transmit diversity is the main application of the invention.
In the case of a single channel consisting of several paths, it is known how to best isolate the repetitions of one and the same signal received by way of the various paths, although these offset repetitions arrive in a superposed manner at the receiver.
For this, a method called rake reception is known, when each sample is processed by spectrum spreading.
Spectrum spreading was initially used to combat interference and/or to multiplex several communications. Spectrum spreading actually makes it possible to combat extra-cellular scrambling, and allows the multiplexing of the various users: the latter share the same frequency band and are differentiated by their spreading sequence, the code, as explained hereinbelow.
The spreading is therefore carried out by multiplying each symbol transmitted by a string of chips, forming a code of length equal to the spreading factor. The code is not necessarily the same for each symbol. The frequency band required to transmit such a signal is increased by a factor F, hence the name “spread spectrum”.
The spreading therefore consists in going from the symbol rate to the chip rate, which is faster, the ratio of the two being dubbed the spreading factor (F), and generally being equal to 2k. The duration of a symbol being T, that of a chip being Tchip, we have T/Tchip=F.
Finally, each chip is shaped by a pulse Pchip(t) in general a Nyquist root filter.p0=∫|pn(t)|2dt=F*∫(pchip(t))2dt 
The transmission channel is customarily regarded as a sum of Diracs, that is to say as one and the same signal repeated and superposed on itself several times, with several variable offsets that are in practice much shorter than the duration of a spread symbol.
Through rake reception it is known how to satisfactorily isolate these various superposed repetitions of the signal at the level of the mobile, by virtue of a method for identifying the repetition of peaks in the superposed signal, and for determining the start of each superposed signal.
More precisely, the signals transmitted consisting of a string of symbols each coded as a series of very fast chips, a filter matched to the coding is applied each time that the start of a sequence corresponding to the coding of a symbol is detected.
This matched filter is therefore applied several times to similar signals corresponding to various paths, offset with respect to one another. With each application of the matched filter, the offset sequences will give rise to only fairly weak noise.
Thus, as represented in FIG. 1, the so-called RAKE technique or rake receiver uses the coding to isolate, in a superposed signal 10, signals 20, 30, 40 each corresponding to one and the same coding of one and the same symbol, for several paths transmitting a symbol Sn.
One and the same signal corresponding to the coding of one and the same symbol Sn is thus isolated several times so as to obtain several evaluations h1Sn, h2Sn, h3Sn . . . of the value of the symbol Sn, each time affected by a transmission factor h1, h2, h3 . . . corresponding to the effect of the particular path.
A channel-matched filter is applied to the values h1Sn, h2Sn, h3Sn . . . obtained, the function of this filter being to deliver a series of evaluations of sn, the effect of whose p paths h1, h2, h3, . . . has been favorably combined. The latter matched filter is estimated by the mobile according to a technique that will not be detailed here.
For each delay associated with one of the paths selected a correlation is performed through the conjugate of the code modulating the symbol, the result of the correlation is multiplied by the conjugate of the estimate of the amplitude of the path, and the various results are summed.
Thus, the rake receiver is a matched filtering, simplified in that: the approximation is made that the channel is formed of a finite number of paths delayed with respect to one another (the impulse response of the channel is therefore formed of a string of Dirac pulses). The matched filtering is done only by taking account of these paths.
A method dubbed Space Time Transmission Diversity (STTD) for best exploiting the information carried by the samples transmitted by way of several antennas and arriving at one and the same mobile in a superposed manner is thus also known.
The concept of STTD has been presented [3] as applying to a transmission of modulated symbols in a channel with frequency-unselective impulse response, (i.e., a complex multiplicative constant), the transmitter consisting of two sensors. The transmission chain is represented in baseband, that is to say after passing through the matched filters and sampling.
A typical example of implementing STTD may be summarized thus: the symbols are grouped in twos. While the first antenna is transmitting the symbols in the order in which they arrive (as it would do if the transmission had occurred on a single antenna), the second transmits the complex conjugates of the same symbols, but in a modified order, and moreover with a change of sign every other time: the second antenna transmits −sn+1*, sn* (* signifies complex conjugate) while the first is transmitting sn, sn+1. Likewise for the subsequent symbols, sn+2, sn+3 etc.
It may be shown that, if the estimation of the channels is perfect on reception (the problems of channel estimation are outside our concern), the expression for the symbols before detection is similar to that which would be provided by a diversity receiver of the “Maximal ratio receiver combining” type [3], the signals being transmitted on a single antenna, and received on two antennas, the propagation channels linking the single transmitter to each receiver being the same ones respectively as those linking each of the transmission antennas to the single receiver of the STTD.
To summarize, the symbols to be transmitted are considered pairwise, they are transmitted while inverting the symbols of the pair on one antenna by inverting the signs of chosen symbols, and transmitting the conjugate of chosen symbols, so as to obtain a 2×2 transmission matrix which is orthogonal.
Thus, it is easy to solve a 2×2 linear equation which, because the mobile knows the transmission functions of the various paths, makes it possible to deduce therefrom independently the transmitted and superposed initial symbols sn and sn+1 of the simultaneous transmission by two antennas. The maximum information transmitted by these two antennas is therefore derived by harnessing the transmission by two antennas in terms of transmission efficiency.
This method has already been generalized to a larger number of antennas, whether the orthogonal transmission matrix be square or otherwise.
The matrix may not be strictly orthogonal, but give by multiplication with its conjugate transpose a matrix which is substantially diagonal, that is to say which exhibits small and/or sparse values off the diagonal, this being acceptable in certain cases.
The symbols may be grouped in p-tuples, other than pairs. The number of antennas n may be different from the number p of symbols per group.
These p-tuples are transmitted by the transmissions, k possibly being different from p and from n, a substantially orthogonal matrix can be obtained which is not necessarily square.
The equations are typically the following in the case of two antennas and of transmission in pairs of symbols:
Let {sn} be the string of symbols to be transmitted. The first antenna will transmit {sn} in this order, while the second antenna is transmitting the same symbols in a modified order. The symbols are grouped in twos.
For example, the second antenna transmits −s2n+1*, s2n* (* signifies complex conjugate) while the first is transmitting s2n, s2n+1. To streamline the notation, and since the parity of the first index of the two grouped symbols is of no importance, they will subsequently be denoted sn and sn+1.
The baseband representation of the system is considered, and the channels involved are considered to group together the effects of the transmission/reception filters and also of the propagation. Let h1 (respectively h2) be the channel linking the first (respectively second) antenna to the mobile.
We haveyn=h1sn−h2sn+1*+bn  (1)yn+1=h1sn+1+h2sn*+bn+1  (2)where bn denotes the noise of the receiver at time n.
The second equation is conjugated:yn+1*=h1*sn+1*+h2*sn+bn+1 
After passing through the filters matched to the various channels, we have:yn1=(|h1|2sn−h2h1*sn+1*)+h1*bn  (3)yn2=(h2h1*sn+1*+|h2|2sn)+h2bn+1  (4)
By adding (3) and (4) we obtain:(3)+(4)=sn(|h1|2+|h2|2)+h1*bn+h2bn+1  (5)
By interchanging the roles of h1 and of h2 in (3) and after linearly combining the results, we likewise have:sn+1(|h1|2+h2|2)+h1bn+1*−h2*bn  (6)
It is therefore seen that, to within a sign on one of the noise terms in (6)—this does not change the power of the noise—everything takes place as if the data to be transmitted, s, were transmitted in succession by a single antenna, were filtered by two channels h1 and h2, and were received by a receiver performing optimal combining of the data received at each symbol time.
The estimation of the channels, prior to the matched filtering, is assumed here to be perfect. The estimation of the channels will not be detailed here.
It has been proposed that the techniques of STTD and of rake reception recalled hereinabove be combined.
Thus, the 3rd Generation Partnership Project (3GPP) [4] has adopted STTD in Universal Mobile Telecommunications System (UMTS) Frequency Division Multiplex (FDD).
It has been proposed that these two methods be associated in the case where a multitude of paths are superposed with a coding of the symbols and where transmission takes place via two diversity antennas.
On each channel, the ripples corresponding to the superpositions of simultaneously coded symbols are firstly separated so that each path corresponding to one and the same instant of transmission is separated (rake reception).
Then a decoding is applied to each ripple of chips (despreading) to obtain values of superposition of two symbols transmitted simultaneously by the two antennas. Stated otherwise, each superposition of symbols is despread. Then, these symbols being transmitted in pairs according to an orthogonal transmission matrix, the orthogonality of the matrix is exploited so as to de-superpose the initial samples.
Stated otherwise, the paths are separated by detecting peaks (step 1 in FIG. 2), then the signals carried by the peaks are decoded pathwise (despreading), doing so twice in succession (step 2 in FIG. 2), and the symbols of the pair are separated by exploiting the orthogonality of 2×2 transmission matrix corresponding to these two successive instants (step 3 in FIG. 2).
The information which is repeated through the presence of various paths is therefore firstly separated (rake reception) and the equations stemming from the orthgonality of the transmission matrix are applied so as to deduce therefrom the two (STTD) initial symbols of each pair.
The orthogonality of the transmission matrix is exploited several times, corresponding to the various isolated paths. It will be noted that in order to do this, the time offsets between paths are preferentially very small compared with the spreading of a symbol.
In this case, the demodulation of STTD type is done path by path, and the results of the calculations relating to each path are added together for each of the STTD symbols.
Stated otherwise, the STTD technique is generalized to frequency-selective channels, via a concept of rake receiver [1], preferably under the assumption that the length of the impulse response of the propagation channel is much shorter than the duration of a symbol.
The equations corresponding to such a combination of methods are presented hereinbelow by way of detail.
The effect of the propagation channel is distinguished therein from that of the filter shaping the symbols transmitted. The propagation channel is in both cases a complex multiplicative constant. This presentation does not feature in [3].
It is noted that the spreading of symbols by codes, which is characteristic of any American Mobile Radio Corporation (AMRC) type system, including UMTS FDD, is merely one particular type of modulation applied to symbols and in which the code applied to the symbol transmitted plays the role of the shaping filter. These equations therefore apply to this type of transmission, where a single code is transmitted at a time, however.
The propagation channel linking the first (respectively second) antenna to the mobile is denoted h1 (respectively h2). These are complex constants. Let pn(t) (respectively pn+1(t)) be the shaping filter for the nth (respectively n+1st) symbol, transmitted over the interval nT≦t<(n+1)T (respectively (n+1)T≦t<(n+2)T) where T denotes the duration of a symbol. It will be assumed that pn and pn+1 have temporal support approximately limited to [0, T]. Let y(t) be the signal received at the mobile.
We have:y(t)=(h1sn−h2sn+1*)pn(t−nT)+b(t)nT≦t<(n+1)T  (1bis)y(t)=(h1sn+1+h2sn*)pn+1(t−(n+1)T)+b(t)(n+1)T≦t<(n+2)T  (2bis)b(t) denotes the noise of the receiver at time t.
After passing through the filters matched to the various channels and sampling at the symbol rate, we have:yn1=∫(nT,(n+1)T)y(t)h1*pn(t−nT)*dt=(|h1|2sn−h2h1*sn+1*)p0+bn1  (3bis)yn2=∫[(n+1)T,(n+2)T]y(t)h2*pn+1(t−(n+1)T)*dt=(h2*h1sn+1+|h2|2sn*)p0+bn+12 where p0=∫|pn(t)|2dt=∫|pn+1(t)|2dt
We conjugate the second equation:yn2*=(h2h1*sn+1*+|h2|2sn)p0+bn+12*  (4bis)
By adding (3) and (4) we obtain:(3)+(4)=sn(|h1|2+|h2|2)p0+bn1+bn+12*  (5bis)
By interchanging the roles of h1 and of h2 in (3) and after linearly combining the results, we likewise obtain:sn+1(|h1|2+|h2|2)p0+bruit  (6bis)
The estimation of the channels, prior to the matched filtering, is assumed to be perfect since the problems of imperfect estimation are off the subject.
It is seen that the above calculation encompasses the case where pn(t) designates a spreading code shaped by the chip pulse in an AMRC system.
However, it is apparent that when rake reception is combined with the STTD technique, nuisance interference appears.
(It is already known that in a system with orthogonal codes the channel selectivity causes a break in the orthogonality of the codes).
The selectivity gives rise in particular to interference between the simultaneously transmitted symbols.
If two symbols, in particular consecutive ones, are in STTD, it is shown hereinbelow that additional interference occurs, caused by the second STTD symbol as well as by all those multiplexed by the codes transmitted during this second symbol time.
The aim of the invention is therefore mainly to reduce this intersymbol interference. By canceling the interference caused by the terms transmitted in the symbol duration of the second STTD symbol, interference which appears when the channel is frequency selective.
The result is a reduction of around ⅔ in the level of intersymbol interference, for equal complexity of the receiver.
The invention applies to STTD whether the spectrum spreading be carried out by means of mutually orthogonal codes or otherwise, and is not limited, in its main object, to 2×2 cases.
With this aim, the invention proposes a transmission method in which:                n antennas, n greater than or equal to 2, are used to transmit symbols grouped into p-tuples (S1, . . . , Sp), while carrying out, for each p-tuple, k times a simultaneous sending of n symbols, the n symbols consisting each time of values from among S1, . . . , Sp which are in a modified order and are assigned weights chosen selectively from among −1, 0 and 1, and are selectively conjugated, so that k linear superpositions of n symbols are transmitted in such a way as to constitute a transmission matrix of dimensions k×p which is substantially orthogonal;        the n antennas furthermore send each symbol with a spread spectrum coding, the codings being the same on the n antennas at each transmission concerned among the k transmissions;        the transmission being performed over a physical or contrived channel which constitutes substantially a sum of Diracs, so that each superposition of symbols is repeated several times on reception; in which method furthermore:        each superposition of n symbols is extracted with the aid of despreadings on reception several times;        these several extractions are done with the aid of despreadings for each of the k superpositions transmitted so that several estimations of k-tuples are obtained, each k-tuple forming a result of the k×p transmission matrix;        the orthogonality of the k×p matrix is exploited several times corresponding to each of the several k-tuple extractions, so that an estimation of the p-tuple (S1, . . . , Sp) and therefore of each symbol S1, . . . Sp is obtained several times;        and an estimation of each symbol S1, . . . , Sp is deduced, sharpened on the basis of the symbols estimated several times,characterized in that k is an even number, and that the symbols are coded according to k successive codes corresponding to the k transmissions, the k codes consisting of k/2 pairs of codes which satisfy, in each pair, the property according to which a code of the pair is time-inverted and conjugated with respect to the other code of the pair, and possibly multiplied by a complex constant of modulus 1.        